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United States Patent |
5,010,121
|
Yeates
,   et al.
|
April 23, 1991
|
Production of aqueous-based fluoropolymer compositions
Abstract
Production of an aqueous-based composition comprising a fluoropolymer and a
vinyl copolymer by (1) preparing a non-aqueous solution of a fluoropolymer
comprising repeat units derived from at least one fluorolefine and repeat
units bearing chain-pendant disperser groups imparting
water-emulsifiability or water-solubility to the fluoropolymer; (2)
converting the non-aqueous solution formed in step (1) into an
aqueous-based emulsion or solution of said fluoropolymer; and (3)
polymerising at least one vinyl monomer to form a vinyl polymer in the
presence of said aqueous-based emulsion or solution from step (2).
Inventors:
|
Yeates; Steven G. (Macclesfield, GB2);
Padget; John C. (Frodsham, GB2)
|
Assignee:
|
Imperial Chemical Industries PLC (London, GB2)
|
Appl. No.:
|
280879 |
Filed:
|
December 7, 1988 |
Foreign Application Priority Data
Current U.S. Class: |
523/336; 524/458; 524/544; 525/276; 525/326.2; 525/386 |
Intern'l Class: |
C08J 003/00 |
Field of Search: |
524/458,544
525/326.2,386,276
523/336
|
References Cited
U.S. Patent Documents
4036802 | Jul., 1977 | Poirier | 524/458.
|
4487893 | Dec., 1984 | Yamake et al. | 525/326.
|
4647612 | Mar., 1987 | Rands et al. | 524/458.
|
Primary Examiner: Schofer; Joseph L.
Assistant Examiner: Sweet; Mark
Attorney, Agent or Firm: Cushman, Darby & Cushman
Claims
We claim:
1. Process for the preparation of an aqueous-based composition comprising a
fluoropolymer and a vinyl polymer which process comprises:
(1) preparing a non-aqueous solution of a fluoropolymer comprising repeat
units A derived from at least one fluorolefine and repeat units B bearing
chain-pendant ionic and/or non-ionic disperser groups where such groups
impart water-emulsifiability or water-solubility to the fluoropolymer;
(2) converting, by virtue of the effect of said emulsifying or solubilizing
disperser groups, the non-aqueous solution of said fluoropolymer formed in
step (1) into an aqueous-based emulsion or solution of said fluoropolymer,
and
(3) polymerising at least one species of vinyl monomer to form a vinyl
polymer in the presence of said aqueous-based emulsion of solution from
step (2).
2. Process according to claim 1 wherein said fluoropolymer used in step (1)
has been derived from a precursor polymer, which precursor polymer
comprises repeat units A derived from said at least one fluorolefine and
repeat units derived from at least one olefinically-unsaturated monomer
having at least one functional group which becomes chain-pendant in the
precursor polymer, by means of converting at least a proportion of said
chain-pendant functional groups in the precursor polymer to chain-pendant
disperser groups thereby forming repeat units B.
3. Process according to claim 2 wherein said chain-pendant functional
groups of the precursor polymer are selected from hydroxyl, amino, and
epoxy groups.
4. Process according to claim 1 wherein said disperser groups comprise
groups which are anionic in nature.
5. Process according to claim 4 wherein said anionic groups are carboxylate
salt groups.
6. Process according to claim 5 wherein said carboxylate salt disperser
groups are provided in step (1) by:
(a) reacting a non-aqueous solution of a precursor polymer, comprising
repeat units A derived from at least one fluorolefine and repeat units of
a hydroxyl-functional olefinically-unsaturated comonomer providing
chain-pendant hydroxyl groups in the precursor polymer, with a dibasic
anhydride of formula:
##STR6##
where M is a divalent organic group, whereby at least a proportion of the
chain-pendant hydroxyl groups are converted to chain-pendant carboxylic
acid-containing groups of formula:
##STR7##
where M is a defined supra; (b) replacing the solvent employed in step
(a), if necessary, with a water-miscible non-aqueous solvent-medium; and
(c) neutralising at least a proportion of the carboxyl groups to form
corresponding carboxylate salt groups.
7. Process according to claim 6, wherein the non-aqueous solvent medium
used in step (a) is itself water-miscible and the replacement step (b) is
not employed.
8. Process according to claim 1 wherein said disperser groups comprise
groups which are non-ionic in nature.
9. Process according to claim 1 wherein the conversion in step (2) is
effected by admixing a non-aqueous solution of said fluoropolymer with
water.
10. Process according to claim 9 wherein the non-aqueous solvent medium
employed for said admixture is the non-aqueous solvent medium resulting,
from step (1).
11. Process according to claim 1 wherein a non-aqueous solvent medium is
employed for the formation of an aqueous-based emulsion or solution in
step (2) which comprises at least one water-miscible solvent selected from
acetone, methyl ethyl ketone, cyclohexanone, methanol, ethanol,
isopropanol, n-propanol, t-butanol, n-butanol, dimethylcarbinol,
tetrahydrofuran, dimethyl formamide, dimethylacetamide, and N-methyl
pyrollidone.
12. Process according to claim 1 wherein a non-aqueous solvent medium is
employed for the formation of an aqueous-based emulsion or solution in
step (2) which comprises at least one vinyl monomer to be polymerised in
step (3).
13. Process according to claim 1 wherein said fluoropolymer comprises
repeat units C derived from at least one olefinically unsaturated monomer
which units are neither derived from a fluorolefine nor derived from a
monomer which is functionalised to provide (ab initio as via further
reaction) chain-pendant disperser groups.
14. Process according to claim 1 wherein said at least one fluorolefine
providing repeat units A is selected from at least one of
tetrafluoroethylene, chlorotrifluorethylene, vinylidene fluoride and
hexafluoropropylene.
15. Process according to claim 1 wherein said vinyl polymer formed in step
(3) is an acrylic polymer.
16. Process according to claim 15 wherein said acrylic polymer is a homo-
or copolymeric acrylic polymer derived from at least one acrylic monomer
of formula CH.sub.2 .dbd.CR.sup.7 COOR.sup.8 where R.sup.7 is H or methyl,
and R.sup.8 is alkyl or cycloalkyl of 1 to 20 carbon atoms.
17. Process according to claim 1 wherein the average particle size of the
polymer particles formed in step (3) is within the range 0.01 to 5
microns.
18. Process according to claim 1 wherein the weight ratio of fluoropolymer
to vinyl polymer in said composition is within the range of from 99/1 to
5/95.
19. An aqueous-based composition produced by a process according to claim
1.
Description
The present invention relates to a process for the preparation of certain
aqueous based compositions containing certain fluoropolymers and vinyl
polymers, and to the compositions so produced.
It is known to employ aqueous-based dispersions of various fluoropolymers
(by which we mean fluorine-containing homo- and copolymers e.g. homo- and
copolymers of fluoroolefines such as tetrafluoroethylene,
trichlorofluoroethylene and vinylidene fluoride) as the basis for the
provision of coatings of exceptionally high quality in terms of
durability, weatherability, chemical, ultraviolet and thermal stability,
stain resistance, and appearance. Such desirable properties stem from the
well- known chemical, solvent and thermal resistance, weatherability, low
friction characteristics, good mechanical properties and water repellancy
of the fluoropolymers themselves. However, coatings derived from such
dispersions mostly require baking at elevated temperatures when being
prepared in order that they should be coherent (because of the inherent
crystallinity of most types of commonly employed fluoropolymers), so that
their use is restricted to locations where heating equipment is available
or its use practicable. Moreover, fluoropolymers tend to be expensive
materials.
It has been proposed to employ aqueous-based dispersions of certain of
these fluoropolymers in combination with acrylic polymers in order to
achieve coherent film-formability at ambient temperatures; this also has
the attraction of providing a cheaper coating material (most acrylic
polymers being much less expensive than fluoropolymers). For example, U.S.
Pat. No. 4141873 discloses aqueous-based dispersions comprising a mixture
of a vinylidene fluoride polymer and an acrylic polymer said to be
film-formable at ambient temperatures. Nevertheless, it is stated therein
that coherent films cannot be prepared from such compositions at ambient
temperatures unless they contain at least 25% by weight (of the total
polymer weight) of acrylic polymer. Moreover, most other fluoropolymers
are poorly thermodynamically compatible with acrylic polymers so that
coherent films cannot generally be formed from aqueous dispersions
containing the two types of polymer unless elevated taking temperatures
are employed (good thermodynamic compatibility in this context is taken to
mean the situation where there is intimate mixing of the fluoropolymer and
acrylic polymer at the molecular level). This has the added disadvantage
that transparent films cannot readily be obtained on ambient or low -
temperature application should these be desired.
We have now discovered how to prepare aqueous-based compositions of
fluoropolymers and vinyl polymers in general (i.e. not merely acrylic
polymers, although these are preferred) in which the fluoropolymers and
vinyl polymers are in extremely intimate admixture in the composition (in
comparison to admixture achieved by simple latex blending) irrespective of
whether the two types of polymer are thermodynamically compatible (they
may or may not be compatible) and irrespective of the level of vinyl
polymer. Thus the fluoropolymer types employed can vary over a wide range
of basic composition (and in particular are not limited to vinylidene
fluoride homo-and copolymers). Many of these compositions yield coherent
film coatings under ambient or low temperature (e.g. up to 60.degree. C.)
application conditions which possess advantageous attributes derived from
the fluoropolymer component (as discussed above) at lower cost; such films
may be transparent if desired (by (by employing appropriate formulations,
i.e. not including opacifying materials). Moreover, the new process allows
a very precise control over the polymer microstructure.
According to the present invention there is provided a process for
preparing an aqueous-based composition comprising a fluoropolymer and a
vinyl polymer which process comprises:
(1) preparing a non-aqueous solution of a fluoropolymer comprising repeat
units A derived from at least one fluoroolefine and repeat units B bearing
chain-pendant disperser groups imparting water-emulsifiability or
water-solubility to the fluoropolymer;
(2) converting the non-aqueous solution of said fluoropolymer formed in
step (1) into an aqueous-based emulsion or solution of said fluoropolymer;
and
(3) polymerising at least one vinyl monomer to form a vinyl polymer in the
presence of said aqueous-based emulsion or solution from step (2).
In the composition produced by the process of the invention (effectively an
aqueous polymer latex composition although non-aqueous solvent material
employed during its preparation may still be present if not subsequently
removed) it is believed that the fluoropolymer and vinyl polymer are
intimately admixed in the aqueous-based composition either (in the case of
a fluoropolymer emulsion being formed in step (2)) by virtue of at least
some (and perhaps substantially all) of the vinyl monomer(s) employed for
the vinyl polymerisation in step (3) becoming absorbed within and/or on
the fluoropolymer particles during polymerisation or (in the case of a
fluoropolymer solution being formed in step (2)) by virtue of the
solubilized fluoropolymer molecules acting as an emulsifying agent in the
polymerising vinyl monomer/polymer mixture; it is also possible (we
believe) that both mechanisms could operate at the same time (depending on
the degree of water solubility of the fluoropolymer, in turn dependent on
the concentration of disperser groups present in the fluoropolymer). In
this way at least some (and perhaps substantially all) of the
polymerisation of the vinyl monomer(s) to the vinyl polymer takes place in
extremely close proximity to the fluoropolymer molecules irrespective of
the inherent thermodynamic compatibility of the fluoropolymer and vinyl
polymer. In a sense, therefore, intimate admixture is forced on the
fluoropolymer and vinyl polymer. It can also be appreciated that the
process of the invention allows very good control of the locus of
polymerisation, and hence close control of the type of polymer
microstructure, to be achieved. For example, polymerisation of the vinyl
monomer soley in the aqueous phase, or within the fluoropolymer particles
themselves, or in intermediate polymerisation loci, can be achieved.
The fluoropolymer comprising repeat units A derived from at least one
fluoroolefine and repeat units B bearing chain-pendant disperser groups
is, for example, prepared as follows. A free-radical addition
copolymerisation of at least one fluoroolefine and at least one
olefinically unsaturated comonomer (which is copolymerisable with the
fluoroolefine(s)) having at least one functional group is carried out
(optionally with other comonomer(s)) to yield a precursor copolymer having
units of the fluoroolefine(s) and also units having the functional
group(s). (By a functional group in this context is meant a pendant
reactive group which can be subsequently converted by one or more
reactions to a disperser group). Suitable precursor fluoropolymers of this
type may be available commercially so that it may not be necessary to
perform this polymerisation oneself. At least some of the chain-pendant
functional groups of the precursor copolymer are then converted by one or
more reactions (as necessary) to disperser groups which as defined above
can impart water-emulsifiability or water-solubility to the fluoropolymer
(repeat units having such chain-pendant disperser groups are herein termed
units B), this usually being done with the copolymer in non-aqueous
solution. Suitable functional groups for incorporation into the precursor
copolymer are e.g. hydroxyl, amino, and epoxy groups; however hydroxyl
groups are particularly preferred.
It is also possible to directly form a fluoropolymer comprising repeat
units derived from at least one fluoroolefine (units A) and repeat units B
bearing chain-pendant disperser groups without the need to first form a
precursor copolymer. This can be achieved by the copolymerisation of at
least one olefinically unsaturated comonomer which already bears a
disperser group(s) or a group that can then be converted to a disperser
group(s) merely by neutralization (e.g. with base or acid). An example of
such a fluoropolymer would be one comprising repeat units derived from at
least one fluoroolefine and at least one olefinically unsaturated
comonomer bearing a carboxylic acid group(s) whereby the pendant carboxyl
groups in the resulting fluoropolymer could be converted to carboxylate
anion disperser groups merely by neutralization with a base.
It is to be understood that the fluoropolymer may also (optionally) contain
repeat units C derived from one or more ethylenically unsaturated monomers
which units are neither derived from a fluoroolefine nor derived from a
monomer which is functionalized to provide (ab initio or via further
reaction) chain-pendant disperser groups. These units C e.g. may be
incorporated by inclusion of such monomer(s) in the polymerisation to form
the presursor copolymer.
Typically the fluoropolymer of the composition made by the claimed process
comprises 30-80 weight % of units A derived from said at least one
fluorolefine, 1-20 weight % of units B bearing chain-pendant disperser
groups, and 0-69 weight % of other units C (i.e. neither derived from a
fluoroolefine nor having pendant disperser groups).
The at least one fluorolefine providing repeat units A is broadly defined
as an olefine having at least one fluorine atom substituent; preferably
the fluoroolefine is a perhaloolefine in which all the hydrogen atoms of
the olefine are substituted with fluorine atoms and optionally other
halogen atoms.
From the point of view of polymerisability and resulting polymer properties
fluoroolefines having 2 or 3 carbon atoms are preferable.
Examples of such fluoroolefines include fluoroethylenes such as CF.sub.2
.dbd.CF.sub.2, CHF.dbd.CF.sub.2, CH2.dbd.CF.sub.2, CH.sub.2 .dbd.CHF,
CC1F.dbd.CF.sub.2, CC1.sub.2 .dbd.CF.sub.2, CC1F.dbd.CC1F,
CHF.dbd.CC1.sub.2, CH.sub.2 .dbd.CC1F, and CC1.sub.2 .dbd.CC1F; and
fluoropropylenes such as CF.sub.3 CF.dbd.CF.sub.2, CF.sub.3 CF.dbd.CHF,
CF.sub.3 CH.dbd.CF.sub.2, CF.sub.3 CH.dbd.CH.sub.2, CF.sub.3 CF.dbd.CHF,
CHF.sub.2 CH.dbd.CHF, and CF.sub.3 CH.dbd.CH.sub.2.
Of the fluoroethylenes and fluoropropylenes listed above
tetrafluoroethylene (CF.sub.2 .dbd.CF.sub.2), chlorotrifluoroethylene
(CC1F.dbd.CF.sub.2), vinylidene fluoride (CH.sub.2 .dbd.CF.sub.2), and
hexafluoropropylene (CF.sub.2 .dbd.CFCF.sub.3) are particularly preferred.
The use of the above exemplified fluoroolefines either singly or in
admixture is of course included within the scope of the present invention.
Examples of suitable olefinically unsaturated comonomers bearing at least
one functional group (for use in the preparation of a precursor copolymer)
which are copolymerisable with fluoroolefines are hydroxy or amino vinyl
ethers of formula:
CR.sup.1 R.sup.2 .dbd.CR.sup.3 O (CR.sup.4 R.sup.5).sub.n Q
where R.sup.1, R.sup.2, R.sup.3, R.sup.4, R.sup.5 may be independently H,
alkyl (preferably of 1 to 5 carbon atoms), or halogen (preferably F, Cl),
n is 2 to 8 and Q is OH or NH.sub.2. Usually R.sup.1 .dbd.R.sup.2
.dbd.R.sup.3 .dbd.R.sup.4 .dbd.R.sup.5 .dbd.H, Q is OH and n is 2 to 6.
Examples of such vinyl ethers are 2-hydroxyethyl vinyl ether,
3-hydroxy(n)butyl vinyl ether, 4-hydroxy(n)butyl vinyl ether,
3-hydroxy(n)propyl vinyl ether, 5-hydroxy(n)pentyl vinyl ether, and
6-hydroxy(n)hexyl vinyl ether. Other possible functional vinyl ethers
include:
2,3-dihydroxypropyl vinyl ether;
3-hydroxy-2,2-dimethylpropyl vinyl ether;
2-methyl-2-hydroxymethyl-3-hydroxypropyl vinyl ether;
2-ethyl-2-hydroxymethyl-3-hydroxypropyl vinyl ether;
3-(hydroxymethyl)-5-hydroxypentyl vinyl ether;
2,2-bis(hydroxymethyl)-3-hydroxypropyl vinyl ether;
1-hydroxymethyl-4-vinyloxymethylcyclohexane;and
2-[2-hydroxyethoxy]ethyl vinyl ether.
Examples of monomers for providing optional units C include: alkyl, aryl,
or cycloalkyl vinyl ethers (or fluoro-substituted derivatives thereof)
such as cyclohexyl vinyl ether, ethyl vinyl ether, n-propyl vinyl ether,
n-butyl vinyl ether, cyclopentyl vinyl ether, and phenyl vinyl ether; and
vinyl esters (or the fluorinated derivatives thereof) such as CH.sub.2
.dbd.CHOCOCH.sub.3 (vinyl acetate), CH.sub.2 .dbd.CHOCOC(CH.sub.3)(C.sub.2
H.sub.5)([CH.sub.2 ].sub.4 CH.sub.3), CH.sub.2 .dbd.CHOCOC(CH.sub.3).sub.2
([CH.sub.2 ].sub.5 CH.sub.3), and CH.sub.2 .dbd.CHOCOPh (Ph is phenyl);
and allyl or fluoroallyl ethers of formula CH.sub.2 .dbd.CHCH.sub.2
OR.sup.6 where R.sup.6 is an allyl or a fluoroallyl group of 2 to 10
carbon atoms, such as tetrafluoroallylether. Other possible monomers
include alpha-olefines such as ethylene, propylene, isobutylene, and
butene-1. Such additional units may usefully be used to control the glass
transition temperature of the fluoropolymer.
By a vinyl polymer in this specification is generally meant any addition
polymer of one or more olefinically unsaturated monomers other than a
fluoropolymer as defined above. The vinyl polymer component of the aqueous
dispersion may be formed by the homo- or copolymerisation of any suitable
radically polymerisable olefinically unaturated compound (or mixture
thereof). Thus, there may be mentioned hydrocarbon monomers e.g.
butadiene, isoprene, styrene, and divinyl benzene; acrylic and substituted
acrylic monomers, e.g. alkyl acrylates, alkyl methacrylates, acrylamide,
methacrylamide, and acrylonitrile; vinyl halides, e.g. vinyl chloride;
vinylidene halides, e.g. vinylidene chloride; vinyl esters; vinyl ethers;
vinyl ketones; and heterocylic vinyl compounds. Multifunctional monomers
such as diallyl phthalate and allyl methacrylate may also be included as
comonomers. The vinyl polymer is particularly an acrylic polymer and
especially a homo-or copolymeric acrylic polymer derived from at least one
acrylic monomer having the formula CH.sub.2 .dbd.CR.sup.7 COOR.sup.8 where
R.sup.7 is H or methyl and R.sup.8 is alkyl or cycloalkyl of 1 to 20
carbon atoms (more preferably 1 to 6 carbon atoms). Examples of these are,
for instance, methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl
methacrylate, and n-butyl acrylate; where the acrylic polymer is a
copolymer, the comonomer(s) may be another acrylic monomer (e.g. as
exemplified above) and/or a non-acrylic monomer such as styrene. The vinyl
polymer is optionally cross-linkable. Such cross-linkability in the vinyl
polymer may be achieved, for example, by including polymerised units of at
least one functional monomer in the polymer, such as for instance
hydroxyethyl methacrylate, glycidyl methacrylate, acrylamide, and
methacrylamide. If such a functional monomer is vinylic in nature, it may
constitute the sole monomer of the vinylic polymer; more usually however
it is a comonomer employed with a nonfunctional vinylic monomer(s)
(particularly one or more of those described above).
The chain-pendant disperser groups of the fluoropolymer may be ionic
(usually anionic) and/or non-ionic in nature and may be introduced by any
suitable technique.
Preferably the disperser groups comprise anionic groups and typically acid
salt groups such as carboxylate. By disperser groups we mean groups that
will cause the fluoropolymer to become well dispersed in an aqueous medium
when incorporated into such a medium. By dispersed we mean that the
fluoropolymer is in an emulsified state or is in aqueous solution. When
emulsified, the fluoropolymer particles can range from quite large in size
(say average particle size of >1 to 10 microns) to colloidal size (say
average particle size of 0.01 to 1 microns). Whether or not a
fluoropolymer is likely to become emulsified or solubilized in the aqueous
medium will depend on the level and type of disperser groups in the
fluoropolymer.
One preferred method for introducing anionic carboxylate salt disperser
groups is that based on a process described in U.S. Pat. No. 4487893 for
producing carboxyl group-containing fluoropolymers. Using such a method, a
non-aqueous solution of a precursor copolymer comprising units of at least
one fluorolefine and units of a hydroxyl-functional comonomer providing
chain-pendant hydroxyl groups is reacted with a dibasic acid anhydride of
formula
##STR1##
where M is a divalent organic group whereby at least part of the
chain-pendant hydroxyl groups are converted to chain-pendant carboxylic
acid-containing groups of formula
##STR2##
where M is as defined above. The solvent (or solvent mixture) at this
stage should be selected taking into account the solubilities of the
fluoropolymer and dibasic anhydride used (this of course applies generally
when undertaking any conversion of a functional group in a precursor
polymer to a disperser group; i.e. the solubilities of the fluoropolymer
and the converting reagent should be taken in account when selecting the
solvent). Suitable solvents include aromatic liquids with boiling point
above 100.degree. C. such as xylene and toluene. Water-miscible (lower
boiling) solvents such as acetone may, however, be used in some
circumstances. As the dibasic anhydride there may be mentioned succinic
anhydride, glutaric anhydride, itaconic anhydride, adipic anhydride,
1,2-cyclohexanoic dicarboxylic acid anhydride,
cis-4-cyclohexane-1,2-dicarboxylic acid anhydride, phthalic anhydride,
1,8-naphthalic anhydride and maleic anhydride. In view of their
reactivity, non-aromatic carboxylic anhydrides are preferred. Particularly
preferred are those in which M is an alkylene group having from 2 to 8
carbon atoms, such as --CH.sub.2 CH.sub.2 --.
The degree of conversion of the hydroxyl groups to
##STR3##
groups may deliberately be made partial or complete depending on the
degree of colloidal dispersability required in the final fluoropolymer (by
employing sufficient of the anhydride to react with some or with all of
the hydroxyl groups; all the anhydride used will be consumed provided an
excess is not present). Generally speaking the CO.sub.2 H equivalent
weight in the fluoropolymer should be in the range 500 to 2500
gmole.sup.-1.
The solvent medium of the solution of fluoropolymer (containing pendant
carboxyl groups) may at this point be replaced by a water-miscible solvent
medium of appropriate hydrophilicity for the subsequent water-dispersion
stage (step (2) of the process of the invention), if it is not already a
solvent medium of appropriate hydrophilicty. Thus e.g. a xylene solvent
medium may be replaced with a water-miscible solvent such as acetone.
The next stage is the neutralization (in the non-aqueous solution) of the
acidic carboxyl groups to form corresponding carboxylate salt
groups--these being the anionic disperser groups in the fluoropolymer.
Suitable neutralising agents include ammonia, tertiary amines such as
triethylamine, and heterocyclic compounds such as pyrrole, pyridine or
pyridazine. Preferably complete neutralization of the acid groups is
effected, although less than complete neutralization can be employed
provided the stability of the subsequently--formed aqueous dispersion of
the fluoropolymer is not adversely affected.
The non-aqueous solution of the fluoropolymer bearing chain-pendant
disperser groups (which can be ionic or non-ionic as discussed above) from
step (1) of the process of the invention is converted in step (2) into an
aqueous-based emulsion or solution of said polymer. This may usually be
accomplished by simply admixing (usually with agitation) a non-aqueous
solution of the fluoropolymer with water. The solvent medium for this step
(which may be a mixture of two or more organic liquids--although a single
organic liquid can be used) provides appropriate solvent and diluent
characteristics and is not necessarily the same as any solvent medium
which might initially have been employed in step (1) (as e.g. when
converting functional groups in a precursor polymer to disperser groups).
In particular it must possess sufficient hydrophilicity for the transition
to an aqueous emulsion or solution (on the admixture with water) to be
effected in a facile manner and with the provision of a final
aqueous-based dispersion (i.e. after vinyl polymerisation in step (3)) of
adequate long-term stability. Therefore, before the admixture with water,
any solvent medium which may have been used (e.g. at an initial stage) in
step (1) (as e.g. when converting functional groups to disperser groups)
and if inappropriate for the transition to an aqueous-based emulsion or
solution in step (2), should be replaced by such a hydrophilic solvent
medium for use in step (2), e.g. by adding the new solvent medium and
removing the initially employed solvent medium by evaporation (preferably
under mild conditions as found e.g. in a rotary evaporator). This solvent
replacement stage may be effected before or after the formation of the
chain-pendant disperser groups on the fluoropolymer (if not already on the
fluoropolymer ab initio) but is normally effected beforehand (and
particularly before neutralization in the case of forming pendant anionic
emulsifier groups) in which case it forms part of step (1).
Of course, it may not be necessary to go through any solvent interchange at
all in step (1). Thus one could use in stage (1) a fluoropolymer in a
dried state (e.g. as a fine powder) irrespective of any solvent used in
its preparation; one could then dissolve this up in a suitable hydrophilic
solvent and then (if necessary) form any pendant disperser groups (e.g. by
neutralization of carboxyl groups to carboxylate anion groups with a
base).
Typical hydrophilic solvent materials for use in the non-aqueous solvent
medium employed in the dispersion step of step (2), which, as explained
supra, may be incorporated as part of step (1), include water-miscible
organic compounds such as ketones such as acetone, methyl ethyl ketone, or
cyclohexanone; alcohols such as methanol, ethanol, isopropanol,
n-propanol, t-butanol, or n-butanol; ethers such as dimethylcarbinol or
tetrahydrofuran, amides such dimethyl formamide or dimethylacetamide; and
heterocyclics such as N-methyl pyrollidone. Further, one or more of the
monomers to be employed in the subsequent vinyl polymerisation of step (3)
may be used as, or as part of, the solvent medium used for the dispersion
in step (2) provided it (or they) has suitable solvent/diluent
characteristics (as discussed above). Examples of such monomers include
n-butyl acrylate and acrylonitrile.
At the end of stage (2) there exists an aqueous-based emulsion or solution
of the fluoropolymer. Effectively this is an aqueous latex or aqueous
solution of the fluoropolymer, although organic solvent is still present
in the aqueous medium. This may optionally be removed (before or after the
subsequent vinyl polymerisation) although it is often allowed to remain.
The average particle size of emulsified fluoropolymer particles which may
be present at the end of stage (2) is usually below 10 microns, and
usually within the range 0.01 to 5 microns, and preferably within the
range 0.01 to 0.5 microns.
In stage (3) of the process of the invention, the vinyl monomer (or
monomers) required for the vinyl polymerisation is (are) admixed with the
fluoropolymer emulsion or solution from stage (2), unless of course it has
itself been employed as, or as part of, the non-aqueous solvent medium of
stages (1) or (2) and is therefore already present.
The vinyl polymerisation of step (3) of the process of the invention may be
carried out by any suitable free-radical initiated polymerisation
technique appropriate to the vinyl monomer(s) that is being polymerised, a
free-radical initiator and (usually) appropriate heating being employed.
For example, the initiator system may be added to a mixture of the
fluoropolymer dispersion and all the vinyl monomer(s) to be polymerised,
or all or some of the vinyl monomer(s) to be polymerised may be added
(e.g. gradually or in stages) to the fluoropolymer dispersion (optionally
containing at least one vinyl monomer for polymerisation) containing
initiator (or a part of the initiator system), with appropriate heating
being applied. Suitable free radical initiators include mixtures
partitioning between the aqueous and organic phases, e.g. combinations of
t-butylhydroperoxide, isoascorbic acid and Fe. EDTA.
The average particle size of the dispersed multi polymer particles at the
end of stage (3) (i.e. of the aqueous-based composition made according to
the process of the invention) is usually below 10 micron, and preferably
within the range 0.01 to 5 microns, particularly 0.01 to 0.5 microns.
An advantageous feature of the present invention is that the vinyl
polymerisation may be performed without any external surfactant being
present (as is normal during conventional vinyl aqueous emulsion
polymerisation). Consequently, the aqueous-based dispersion of the
invention can be surfactant-free (with attendant advantages flowing
therefrom). Naturally, however, where the disperser groups of the
fluoropolymer are anionic in nature (e.g. carboxylate salt-type) the
presence of free acid(s) in the aqueous- based dispersion of the
fluoropolymer before or after vinyl polymerisation should normally be
avoided as it may destabilise the dispersion. (Thus, e.g., a free acid
monomer, such as acrylic acid, should not normally be employed in the
vinyl polymerisation when the fluoropolymer contains anionic chain-pendant
emulsifier groups). However, the presence of small levels of acid (such as
acrylic acid) can sometimes be present if the system is appropriately
bufferred. This restriction does not of course apply when the
fluoropolymer has only non-ionic chain-pendant emulsifier groups. Thus,
aqueous-based polymer compositions produced by the process of the
invention (bearing in mind the above- mentioned proviso) are stable for
long periods of time despite the absence of external surfactants (acting
as emulsifiying agents). If desired, however, minor amounts of external
surfactants may be included in the dispersions (being incorporated e.g. in
the vinyl polymerisation step or subsequent thereto).
The fluoropolymer and/or the vinyl polymer may optionally possess the
facility (by the inclusion of appropriate functionalities therein, and
ensuring an appropriate locus for the vinyl polymerisation as explained
supra) for enabling a certain proportion of graft polymerisation to occur
between the polymers during the vinyl polymerisation. In the case of the
fluoropolymer this may e.g. be realized by employing appropriate
functionalised monomer(s) which provide part of the other units C in the
fluoropolymer. Similarly, one or more of the vinyl monomers may be
appropriately funtionalised to provide this facility.
The fluoropolymer and/or the vinyl polymer may also optionally posses the
facility (by the inclusion of appropriate functionalities therein) for
imparting cross-linkability to the resulting polymer dispersion according
to the invention.
It is believed that, in effect, the fluoropolymer in the aqueous-based
dispersion from step (2) of the process of the invention acts as
surfactant-free seed particles for the subsequent vinyl polymerisation
and/or as water-soluble emulsifying molecules acting at the surface of the
vinyl monomer/vinyl polymer system undergoing polymerisation. In either
case an extremely intimate mixture of the two polymers results. Such an
aqueous composition has significantly improved properties, particularly in
comparison to aqueous dispersion blends of fluoropolymers and vinyl
polymers in which the polymers are inherently thermodynamically
incompatible (as is usually the case). Thus unlike simple blends which may
separate and form hazy films when used as coating compositions, the
compositions of the present invention are highly resistant to separation
and, as a result, many of them are able to form very clear films even at
ambient or low temperatures (provided of course no opacifying additives
are present).
The weight ratio of fluoropolymer to vinyl polymer in the dispersion is
suitably in the range from 99/1 to 5/95, preferably 95/5 to 10/90, and
particularly 90/10 to 15/85.
The minimum film-forming temperature of the aqueous-based compositions of
the invention may vary over a wide range, e.g. within the range
-10.degree. to 120.degree. C. (more usually 0 to 80.degree. C.).
As mentioned above, precursor fluoropolymers having functional groups which
are suitable for conversion into disperser groups (to provide
fluoropolymers as used in the present invention) may be available
commercially, in which case they may be purchased rather than prepared.
However, they can be prepared if necessary by appropriate copolymerisation
of at least one fluoroolefine with at least one copolymerisable comomoner
bearing an appropriate functional group (or groups). Such precursor
fluoropolymers may be prepared by conventional polymerisation in aqueous
emulsion, in aqueous suspension, in solution or in bulk. Polymerisation is
usually carried out in the temperature range -20.degree. to 180.degree.
C., more usually 80.degree. to 150.degree. C., under a pressure usually in
the range 1 to 50 Kg/Cm.sup.2. A free- radical yielding initiator is
normally employed, for example an organic peroxide, azo compound, or a
redox initiator system comprising persulphate (as appropriate to the
polymerisation medium). Optionally a chain transfer agent, for example
methyl cyclohexane, may be used to control the molecular weight. The
polymerisation medium may, for example, be water or an aqueous medium (in
the case of emulsion or suspension polymerisation) or an appropriate
organic solvent (in the case of solution polymerisation). Emulsion
polymerisation is normally performed in the presence of a surfactant(s) as
emulsifier.
For the conversion of the precusor fluoropolymer to a fluoropolymer bearing
chain-pendant disperser groups, the precursor polymer should be dissolved
in a non-aqueous solvent medium. Consequently, the precursor fluoropolymer
must be an organo-soluble material; some fluoropolymers will accordingly
be unsuitable for this purpose and the selection of the comonomer(s) may
be crucial to achieve solvent solubility. If the precursor polymer is
prepared using an aqueous medium it should of course first be separated
from the aqueous phase used in its preparation (in fact it is normally,
before dissolution, a dry product separate from any medium used in its
preparation--even if this is a non-aqueous solvent). This solvent medium
initially employed (typically e.g. xylene or toluene) is often replaced by
a more hydrophilic solvent medium prior to dispersion of the fluoroolefine
(bearing disperser groups) in water as described above.
The fluoropolymer used in the composition invention usually has a number
average molecular weight Mn within the range 500 to 300,000 (more usually
1000 to 150,000). The vinyl polymer usually has Mn within the range 1000
to 500,000.
The aqueous-based composition produced by the process of the invention may
also include, or be subsequently formulated with, various other
ingredients. For example, it may if desired include, or subsequently be
formulated with, ingredients commonly employed in film-forming coating
formulations, such as defoamers, rheology control agents, thickeners,
dispersing and stabilizing agents (usually surfactants), wetting agents,
fillers, extenders, fungicides, coalescing solvents, plasticisers,
anti-freeze agents, and pigments. The composition of the invention may
also if desired contain, or be subsequently formulated with, one or more
other type of polymer (i.e. polymer other than that used in stages (1),
(2) and (3) as defined above). Such an other type of polymer could e.g. be
a condensation polymer such as a polyurethane, an epoxy polymer or a
polyester. For some applications, it is envisaged that the composition
will be in the form of, or will subsequently be used in a formulation to
provide, a paint, and will therefore include materials commonly employed
in paint formulations, such as pigments and other ingredients where
appropriate (extenders, stabilisers, thickeners, coalescing solvents,
defoamers, surfactants, and so on).
The fluoropolymer compositions of the present invention may be used for
coating substrates and can be applied in the same manner as any ordinary
liquid coating materials to the surface of a substrate such as metal,
wood, plastics, ceramic, paper or glass. Any suitable coating method may
be used, e.g. brush, spray, roller or dipping. Coherent film formation may
often be achievable by employing ambient temperature application
conditions although sometimes low temperature heating (e.g. up to
60.degree. C.) may be required. The film coatings from the fluoropolymer
compositions of the invention are of high quality, having the excellent
properties imparted by the fluoropolymer component as well as good gloss
and (if appropriately formulated) transparency.
The present invention is now illustrated by the following examples. Unless
otherwise specified all parts and percentages are on a weight basis.
EXAMPLE 1
A commercially available functionalised fluoropolymer, `Lumiflon` LF916
(Asahi Glass), having hydroxyl and carboxyl functionalities was employed
as a precursor polymer. This polymer is considered to comprise units of
chlorotrifluoroethylene and certain other vinyl ether compounds. The exact
composition of this copolymer was not indicated although the respective
functionality contributions were as follows:
OH=58 mg KOH:OH eq.wt=967.2
CO2H=7 mg KOH:CO2H eq.wt=8014
Approximately 50% of hydroxyl groups in the precursor polymer were
converted to anionic chain-pendant disperser groups as follows. The
precursor polymer was provided as a 65% w/w solution in xylene and 100 g
of this solution, together with further xylene (50 g), was charged to a
stirred vessel under nitrogen. The contents of the vessel were heated
(with agitation) to 110.degree. C. and succinic anhydride (3.36 g) added
along with triethylbenzoyl ammonium bromide (0.168 g; 5% on succinic
anhydride). After 25 minutes at this temperature the reaction was complete
(as noted by infra-red spectroscopy applied to small samples taken
periodically). Half of the pendant hydroxyl groups were therefore
converted to chain-pendant groups of formula
##STR4##
The xylene was removed by rotary evaporation (50.degree. C., 6.5 m Bar).
70.22 g of the resulting resin were dissolved in 1/1 acetone /n-butyl
acrylate solvent medium to give a resin/solvent weight ratio of 60/40
(0.03 g of `Topanol` O, a hindered phenol antioxidant, being included to
stabilise the n-butyl acrylate). All the acid groups in the resin were
then neutralized to carboxylate salt groups by the addition of
triethylamine (4.552 g) to provide the anionic chain- pendant disperser
groups.
An aqueous-based dispersion of this fluoropolymer was prepared by adding
65.37 g of the fluoropolymer solution to water (91.52 g) with stirring
over 30 minutes; this resulted in a blue-tinged dispersion with the
following properties:
______________________________________
average particle size 0.091 micron
pH 8.67
solids content (excluding butyl
22.97% w/w
acrylate)
Brookfield Viscosity (Spindle 3
150 centipoise
Setting 12)
Minimum Film Forming Temperature
<5.degree. C.
______________________________________
A 50/50 w/w fluoropolymer/acrylic polymer aqueous-based dispersion
according to the invention (the acrylic polymer being a methyl
methacrylate/n-butyl acrylate copolymer; 66/34 weight-ratio) was prepared
as follows. 120 g of the above aqueous-based dispersion of fluoropolymer
(containing 27.57 g fluoropolymer and 9.19 g n-butyl acrylate) were
charged to a stirred vessel and heated to 75.degree. C. To the vessel was
added t-butylhydroperoxide (0.14 g; 0.5% on monomer). A mixture of
isoascorbic acid (8.27 g; 0.3% on monomer; 1% aqueous solution) and water
(31.73 g), and methyl methacrylate (18.38 g; plus a drop of triethylamine)
were then added dropwise over 30 minutes. The polymerisation was continued
for about 30 minutes and the system was sparged to removed acetone and any
residual vinyl monomer. An extremely stable aqueous-based latex of the
fluoropolymer and acrylic polymer resulted, with <1% coagulum being
observed. The properties of the composition were:
______________________________________
average particle size 0.049 micron
pH 8.04
solids content 31.2% w/w
% monomer conversion 100%
Brookfield Viscosity (Spindle 3
<100 cps
Setting 12)
Minimum Film Forming Temperature
21.4.degree. C.
______________________________________
Clear films were cast from this emulsion of small particle size.
EXAMPLE 2
To a reaction vessel was charged 1421 g of `Lumiflon` LF916 (stripped of
xylene) and 948 g of acetone. Dissolution of the precursor polymer in the
acetone to give a 60% w/w solution was carried out by heating to
50.degree. C. under mild agitation. The vessel contents were then heated
to 60.degree. C. under nitrogen, and succinic anhydride (75.53 g) and
triethylbenzoyl ammonium bromide (3.68 g) added. After 3 hours at this
temperature the reaction was complete (as noted by infra-red spectroscopy
applied to periodically taken samples), whereby half the pendant hydroxyl
groups were converted to chain-pendent groups of formula
##STR5##
Triethylamine (74.35 g) was added to the acetone solution to neutralize
the available carboxyl groups to carboxylate salt disperser groups. After
30 minutes, the neutralized polymer solution was added to water (4000 g)
over a 1 hour period with stirring. The acetone was then removed to yield
a translucent dispersion of the polymer in water having the following
properties:
______________________________________
average particle size 0.025 micron
pH 8.0
Solids content 26.82% w/w
Minimum Film Forming Temperature
16.degree. C.
______________________________________
A 50/50 w/w fluoropolymer/acrylic polymer aqueous dispersion according to
the invention (the acrylic polymer being a methyl methacrylate/n-butyl
acrylate copolymer; 80/20 weight-ratio) was prepared as follows. 150 g of
the above aqueous-based dispersion of fluoropolymer were charged to a
stirred vessel and heated to 76.degree. C. To the vessel was added
t-butylhydroperoxide (0.20 g; 0.5% on monomer). A mixture of isoascorbic
acid (4.02 g; 0.3% on monomer; 1% aqueous solution) and water (95 g), and
methyl methacrylate (32.18 g) and n-butyl acrylate (8.05 g) (plus a drop
of triethylamine) were separately added dropwise over 30 minutes. The
polymerisation was continued for about 30 minutes after which the vessel
was cooled to room temperature. A stable fluoropolymer/acrylic polymer
dispersion resulted having <1% coagulum. The properties of the composition
were:
______________________________________
average particle size 0.040 micron
pH 8.6
Solids content 27.5% w/w
% monomer conversion 98%
Minimum Film Forming Temperature
35.degree. C.
______________________________________
Films were cast from this composition (drying 100.degree. C./3 minutes) and
had the following properties shown below. The properties of films cast
from (a) a blend of the precursor fluoropolymer xylene solution and a
solution of the separately-prepared acrylic polymer (Mn 50000) in xylene
(50/50 w/w) (Example C3) and (b) a solution of the separately-prepared
acrylic polymer only in xylene (Example C4) are given for comparison
purposes.
______________________________________
60.degree. Gloss Retention
(1000 hours QUV
Composition of accelerated weathering
Example No. Film Clarity
tester*)
______________________________________
2 Clear 90%
C3 Opaque 65%
C4 Clear 30%
______________________________________
*12 hour cycle (8 hours UV at 50.degree. C. plus 4 hours condensation at
40.degree. C.)
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